1. Field of the Invention
The present invention relates to a resin composition for toner which is employed for developing an electrostatic charge image in electrophotography or the like, and a method of preparing the same. More particularly, it relates to a resin composition for toner capable of providing toner which is suitably applied to a fixing system using a heating roller in a wide fixing temperature range with excellent offset resistance and blocking resistance, and a method of preparing the same.
2. Description of the Background Art
In general, a dry developing system is employed for developing an electrostatic charge image in electrophotography or the like. Such a dry developing system utilizes a two-component fine-powder developer containing toner, which is prepared by dispersing a colorant such as carbon black in resin, and a carrier of iron powder or glass beads, or a one-component fine-powder developer prepared by magnetizing toner itself.
In this dry developing system, the toner which is charged by friction adheres to an electric latent image provided on a photoreceptor by electric attraction, thereby forming a toner image. Then the toner image formed on the photoreceptor is transferred to paper, and thereafter the as-transferred toner is fixed to the paper by a heating roller whose surface is formed to have mold releasability with respect to the toner, to form a permanent visible image.
When the heating roller is employed as fixing means, the toner must have excellent offset resistance so that the toner will not adhere to the heating roller, excellent fixability so that the toner strongly adheres to the paper, and excellent blocking resistance so that toner particles are hardly flocculated. In particular, the toner must exhibit excellent offset resistance over a wide fixing temperature range.
In order to satisfy the aforementioned requirements, there has been proposed a method of employing a low molecular weight component and a high molecular weight component for expanding the range of molecular weight distribution of a resin composition for toner, as disclosed in Japanese Patent Laying-Open No. 50-134652 (1975), 56-16144 (1981) or 56-158340 (1981). In order to widen the fixing temperature range and improve offset resistance in such a method, however, it is necessary to increase the ratio or molecular weight of the high molecular weight component. This may lead to reduction in fixing strength and crushability of the toner.
Japanese Patent Laying-Open No. 63-66563 (1988) discloses a resin composition for toner which is prepared by mixing a low molecular weight ester compound into resin. According to this method, however, offset resistance of the as-obtained toner is rather reduced although low-temperature fixability thereof is improved.
Further, each of Japanese Patent Publication No. 52-3304 (1977) and Japanese Patent Laying-Open No. 58-59455 (1983) discloses a structure obtained by adding polyolefin wax to a styrene polymer for serving as a mold release agent for suppressing adhesion to a heating roller. Due to inferior compatibility between the polyolefin wax and the styrene polymer, however, the polyolefin wax cannot sufficiently serve as a mold release agent. Further, offset resistance of the as-obtained toner is reduced since the polyolefin wax is easily liberated from the toner. In addition, toner particles are easily flocculated during preservation or copying, due to inferior fluidity of the polyolefin wax.
On the other hand, Japanese Patent Laying-Open No. 56-154741 (1981) relating to toner for a pressure fixing method proposes employment of a graft polymer containing 15 to 70 percent by weight of an ethylene-vinyl acetate copolymer and 85 to 30 percent by weight of a vinyl polymer as a resin composition for toner. However, the pressure fixing method is absolutely different from the heating roller fixing method in conditions such as temperatures and pressures for fixing, with absolutely different performance required for the toner. When the toner disclosed in this literature is applied to the heating roller fixing method, therefore, the toner is deteriorated in crushability, fluidity, blocking resistance and environment dependency due to the high content of the ethylene-vinyl acetate copolymer.